Consumers have become accustomed to the rapid channel change capabilities of present-day NTSC (National Television Standards Committee) television receivers. Such television receivers change channels by changing a tuner frequency (i.e., tuning), and acquiring a television signal modulated onto the tuner frequency (i.e., synchronizing). The predominant time delay in an NTSC television receiver channel change operation is the time required to change the tuner frequency. Acquiring picture synchronization is extremely rapid, since horizontal and vertical synchronization information occurs frequently (i.e., 63 microseconds and 33 milliseconds, respectively) in an NTSC television signal.
In the relatively near future, the type of television receiver used by consumers will be radically changed. Specifically, future television receivers are expected to be implemented substantially in accordance with the transmission standards established by the Advanced Television Standards Committee (ATSC). A similar standard in the European Digital Video Broadcasting (DVB) standard. A compressed digital video system is described in the ATSC digital television standard document A/53, incorporated herein by reference. Moreover, the Moving Pictures Experts Group (MPEG) has promulgated several standards relating to digital data delivery systems. The first, known as MPEG-1, refers to ISO/IEC standards 11172 and is incorporated herein by reference. The second, known as MPEG-2, refers to ISO/IEC standards 13818 and is incorporated herein by reference.
Unfortunately, changing channels in a television receiver conforming to the ATSC-like standards is inherently a much slower process than the above-described NTSC process. Specifically, an ATSC television receiver must sequentially perform many steps to acquire a television signal and produce a series of images from that signal. First, a tuner frequency must be adjusted to a frequency allocated to the channel of interest. Second, the VSB or QAM demodulator must perform a carrier recovery process to begin to produce valid output data. Third, the vestigial sideband (VSB) or quadrature amplitude modulated (QAM) demodulator must acquire data field and data segment synchronization information. Fourth, a transport decoder must acquire packet synchronization information, and then decode so-called program map tables (PMT) and program access tables (PAT) to deliver video and audio data to the respective decoders. Fifth, video rate buffers must be loaded with the video data corresponding to the channel of interest. Sixth, the video decoder begins to perform variable length decoding (VLD), converting the bit stream into instructions and data suitable for use in subsequent decoding steps. Seventh, a video decoder must wait for the occurrence of an intra-coded frame (I-frame) in the data stream before picture decompression can begin. The rate of I-frame occurrence is controlled by a television broadcaster's compression encoder, though the rate will likely be one I-frame per each 12 frame group of pictures (GOP) transmitted. Thus, the average latency for video acquisition alone will be on the order of six frames (approximately 200 milliseconds) and the worst case latency for video acquisition alone will be on the order of 12 frames (approximately 400 milliseconds). It is believed that such a high latency channel change operation will be unacceptable to consumers.
Therefore, it is seen to be desirable to provide a method and apparatus for providing a rapid, or seemingly rapid, channel change or channel acquisition capability in a ATSC television receiver.